The performance of a communication link between a satellite and a mobile platform (i.e., aircraft, ship, train, truck, etc) is influenced by many factors. Most prominently is the effective isotropic radiated power (EIRP) of the satellite antenna, in addition to the slant range, rain loss, and Gain over noise temperature (G/T) of the receive antenna being used to form the link with the satellite. The EIRP of satellite transponders typically varies across a coverage region, as does the slant range and rain loss. In addition, some antennas, such as a planar phased array antennas, exhibit a large G/T reduction with increasing scan angle. The antenna scan angle, and hence the receive G/T, can vary significantly with the location and attitude (pitch, roll and yaw) of the mobile platform. The result is that the performance of a communication link from the satellite to the mobile platform can vary over a large range depending on whether the mobile platform is in a favorable or unfavorable location and attitude (relative to the satellite) in a coverage region. In practice, this performance variation can be as large as 10× (10 dB) over a coverage region. Performance variation can become even larger when the mobile receivers use different size aperture antennas. Larger antennas provide better link performance. Link performance can be defined in many ways. In this context, it is defined as the maximum data rate at which the communication link can operate with a given bit error rate (BER), as described further in the following paragraphs.
The present discussion refers to mobile platforms that are in less favorable locations and operating with smaller aperture antennas as being “disadvantaged”, while mobile users in favorable locations and operating with larger antenna apertures are referred to as “advantaged”. One factor that determines the performance or degree of favor for a particular location in satellite coverage region is illustrated with reference to FIG. 1. The EIRP variation for a typical Ku-band geostationary satellite transponder (e.g. Telstar 6 at 63° west longitude) is shown in FIG. 1. Notice that there is about 2 dB variation across the continental United States (CONUS) coverage area. As mentioned previously, other factors can cause a large change in performance across a coverage region. Table 1 below shows the effect of slant range and antenna scan angle loss across CONUS. The scan angle loss for a planar phased array antenna manufactured by The Boeing Company is approximately equal to cos1.2(θ), where θ is the elevation scan angle to the target satellite, measured with respect to an axis extending perpendicular to the planar aperture.
LocationFree Space Loss (dB)Antenna Scan Loss (db)Seattle, WA205.83.5Brownsville, TX205.30.7Delta (dB)0.52.8
An analysis can be performed to determine the highest date rate at which a communication link may be operated with a specified bit error rate (BER). Further to the present discussion, a communication link is considered to be “closed” or “available” when it achieves less than some threshold BER. For this discussion, the threshold BER is assumed to be 1E-9, or one erred bit for every billion received. Any excess received power beyond that required to “close” the link is referred to as “margin”. In the present discussion, the term “data rate” will be used, however, an even more accurate term for “data rate” is “information rate”, which is the available data rate after removing forward error correction (FEC) and other overhead information. Thus, the terms “data rate” and “information rate” will be used interchangeably throughout the following discussion, although “information rate” is, strictly speaking, a more accurate term to describe the available data rate of a communication link. A user that is in a favorable location within a coverage region is one that can close his communication link at a higher data rate. Alternatively, a challenged user, or a user in a less favorable location within a coverage region, will only be able to achieve communication link closure using lower data rates.
Referring to FIG. 2, this figure shows contours of the highest data rates at which links can be “closed” using a Ku-band transponder on Telstar 6 using a Boeing planar phased array receive antenna having an active aperture measuring 17 inch (43.18 cm)×24 inch (60.96 cm) and having 1500 elements mounted flat on the crown of an aircraft flying in level attitude. The analysis used to generate this is highly sophisticated and includes the effect of adjacent satellite interference. Adjacent satellite interference is caused by the use of small aperture mobile antennas and the elongation of the phased array antenna beam that occurs with increasing scan angle. Adjacent satellite interference causes further variations in the link performance across a coverage region. The contours are generated by performing a link analysis at equally spaced geographic grid points and constructing performance contours. At each grid point the aircraft is rotated 360° in heading to find the worst case heading. The maximum data rate at which the link can be closed for the worst case heading is shown in FIG. 2. Within region A, the maximum channel data rate at which the link can be closed is 12 Mbps. Within region B a maximum channel data rate of 10 Mbps can be used. Within region C a maximum of 8 Mbps, within region D, 6 Mbps; within Region E, 4 Mbps; and within region F, a maximum of 2 Mbps can be utilized.
A communication system using a single forward link data rate would have to operate at a data rate commensurate with the most disadvantaged mobile platform in the coverage region. By “forward link” it is meant a signal from a satellite to the mobile platform. Typically, system designers select the highest data rate at which the communication link can be closed to the most disadvantaged mobile platform in a given coverage region. For example, suppose that one wishes to choose a single data rate for communication across CONUS. FIG. 2 shows that the 6 Mbps contour covers nearly all of CONUS except for a tiny slice of land in northern North Dakota. Therefore, 5-6 Mbps would be a good choice for CONUS operation. However, there are regions within CONUS at which the link can be closed at twice this data rate (i.e., 12 Mbps). Therefore, operation with a single data rate is very inefficient because there are typically many advantaged mobile platforms that can operate at much higher data rates. In other words, the advantaged mobile platforms have large excess margins in their forward links which is being wasted when a single, lower, data rate channel is used to service all mobile platforms operating within a given coverage region.
There is also a “coverage vs. capacity” tradeoff associated with the selection of a single forward link data rate. A low data rate (i.e., low capacity) permits the link to be closed over a wider coverage area. In contrast, a high data rate is only available in a small coverage area. FIG. 3 shows the difference in coverage area for 2 Mbps and 8 Mbps. The 2 Mbps region is greater than 3 times the area of the 8 Mbps region. If multiple data rates could be employed, then both wide coverage and high capacity could be achieved. This is not possible when operating with a single data transmission rate.
Another problem that must be considered is that the most disadvantaged mobile platform typically operates with little or no margin, which means that the communication link is not very robust. For example, suppose a disadvantaged mobile platform (e.g., an aircraft) must operate with a forward link data rate chosen so that the communication link with the satellite is barely closed. Now suppose that the aircraft banks away from the satellite during flight. If the aircraft is using a planar phased array antenna mounted flush on the crown of the aircraft, then the scan angle to the satellite will increase and the G/T will decrease. This can cause a loss of the communication link. Similarly, the aircraft could stray outside the designated coverage region and lose its communication link.
In summary, problems with the existing “single data rate” approach include capacity inefficiency and lack of robustness (i.e., lack of margin). The lack of robustness can cause a loss of the communication link if the operational environment is adversely affected such as by adverse weather. Rain loss as well as standing water and/or ice on the aircraft receive antenna radome or aperture also represents situations where the lack of robustness of a single data rate approach can compromise the ability to achieve and maintain link closure with a mobile platform. The lack of margin also makes it more difficult to initially acquire the target satellite if the antenna on the mobile platform is not pointed precisely at the target satellite.
One method for addressing the above-described problem of managing communications links with a number of different mobile platforms capable of operating at varying data rates within a given coverage region could involve the use of a single carrier that is continuously switched between different data rates. Data packets sent to advantaged mobile platforms could be sent at a higher data rate than to disadvantaged mobile platforms. Making such “on-the-fly” data rate changes requires significant time to synchronize the mobile platform RF receiver to each burst of data that is received at different data rates, resulting in a loss of efficiency. Also, burst mode receivers are far more complex, expensive, and provide reduced performances compared with continuous mode receivers, which are used in the invention.
Another approach for solving the above-described problem in addressing multiple mobile platforms capable of communicating at different data rates is the well known “fade mitigation” method. This method is employed with the Advanced Communication Technology Satellite (“ACTS”) operated by the National Aeronautics and Space Administration (NASA). This method involves reducing the information transmission rate during a rain fade. More specifically, it works by adding forward error correction (FEC) coding during a fade event and removing it during clear weather conditions. Since the bit rate is constant, the addition of FEC overhead reduces the information rate during a fade event and increases it in clear weather. Such a method could be used to efficiently service advantaged and disadvantaged mobile platforms, except for the fact that this approach has insufficient dynamic range. As previously mentioned, the dynamic range between advantaged and disadvantaged mobile terminals is typically more than 10 dB in a coverage region. FEC will provide at most only about five dB of dynamic range. Thus, this method would be unsuitable for use in connection with mobile platforms operating within a relatively large coverage region such as CONUS.